1. Field
The present disclosure relates generally to wireless communication networks, and more particularly, to various concepts and techniques for handing off an access terminal at the network layer in an access network.
2. Background
Wireless communication systems are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. These systems commonly employ an access network capable of connecting multiple access terminals to a wide area network (WAN) by sharing the available network resources. The access network is generally implemented with multiple access points dispersed throughout a geographic coverage region. The geographic coverage region is generally divided into cells with an access point in each cell. The cell may be further divided into sectors. The access point generally includes one transceiver function for each sector in the cell. The transceiver function provides an air interface attachment point for access terminals in the sector.
The access network may also include one or more network functions. In a typical configuration, the network function acts as a controller for any number of transceiver functions and performs various tasks including allocating, managing and tearing down resources for the access terminals. The network function also provides an Internet Protocol (IP) layer attachment point (IAP) for the access terminals. All IP packets destined for the access terminal are sent via the IAP. The access network may have a centralized network architecture defined here as multiple network functions supporting multiple access points, i.e., each network function supporting multiple access points and each access point supported by multiple network functions, or a distributed network architecture defined here as dedicated network function for each access point, i.e., each network function supports a single access point and each access point is supported by a single network function.
In addition to providing an IP layer attachment point, the IAP may also responsible maintaining a session state for any number of access terminals. The session state for an access terminal is the state of the access network on the control path between the access terminal and the IAP that is preserved when a connection is closed. The session state includes the value of the attributes that are negotiated between the access terminal and the access network. These attributes affect the characteristics of the connection and the service received by the access terminal. By way of example, an access terminal may negotiate the quality of service (QoS) configuration for a new application and supply new filter and flow specifications to the access network indicating the QoS service requirements for the application. As another example, the access terminal may negotiate the size and type of the headers used in communication with the access network.
In some wireless communication systems, an access terminal in a given sector establishes a connection with an access point by making an access attempt on an access channel of a transceiver function serving that sector. The network function associated with the transceiver function receiving the access attempt contacts the session master for the access terminal and retrieves a copy of the access terminal's session state. On a successful access attempt, the access terminal is assigned air interface resources such as a MAC ID and data channels to communicate with the transceiver function serving the sector. In addition, the IAP is moved to the serving network function, or alternatively, an IP tunneling protocol is used to send IP packets between the IAP and the serving network function.
In some wireless communication systems, once the access terminal establishes a connection with an access point, it listens for other sectors and measures the signal strength of the sectors it can hear. The access terminal uses these measurements to create an active set. The active set is a set of sectors that have reserved air interface resources for the access terminal. The access terminal will continue to measure the signal strength of other sectors and may add or remove sectors from the active set as it moves around the access network. Alternatively, the access terminal can send a report of the signal strength measurements to the access network so that the access network can maintain the active set.
One function of the active set is to allow the access terminal to quickly switch between sectors and maintain service without having to make a new access attempt. The process of switching sectors, either on the forward or reverse link, is often referred to as a “L2” handoff because it constitues a handoff of the access terminal at the link layer. The access network uses the active set to quickly perform L2 handoff by (1) reserving air interface resources for the access terminal in each of the sectors in the active set, and (2) providing a copy of the session state from the session master to each network function serving a sector in the active set.
Another technique that may be used to support quick handoff of the access terminal is to switch sectors without moving the IAP. The process of moving the IAP is often referred to as a “L3” handoff because it constitues a handoff of the access terminal at the network layer. By decoupling the L2 and L3 handoffs, the distance between the IAP and the serving access point may increase as the access terminal moves through the access network. In such a scenario, it may be desirable to move the IAP closer to the access terminal to improve the latency and routing efficiency experienced on the backhaul. If the IAP serves as the session master, then the session state also needs to be transferred.
The L3 handoff has been traditoinally controlled by the access network. This is often referred to as “network based mobility management.” Using the access terminal to control the L3 handoff (“AT based mobility management”) has often been considered too slow for high speed applications, such as voice. However, AT based mobility management has some advantages, such as allowing for a single mechanism for inter and intra technology, or global and local mobility. It also simplifies the network interfaces further by not requiring the network elements to determine when to do L3 handoff.
Accordingly, there is a need in the art for optimizing the L3 handoff to support AT based mobility management. Optimizing the L3 handoff is a useful feature for every system regardless of the network architecture since it simplifies network interfaces and should also improve the seamlessness of the L3 handoff.